1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a method and an apparatus for driving a memory of a liquid crystal display device capable of reducing the number of frame memories.
2. Description of the Related Art
In general, liquid crystal display devices control the light transmissivity of liquid crystal cells in accordance with video signals to display pictures.
In such liquid crystal display devices, an active matrix type of liquid crystal display device in which a switching device is formed in each liquid crystal cell is suitable for displaying motion pictures due to an active control. The switching device used in the active matrix type of liquid crystal display device generally is a thin film transistor (hereinafter referred to as a ‘TFT’).
A liquid crystal display device, as shown in the following formulas 1 and 2, has a disadvantage that its response time is slow due to its properties such as, for example, the unique viscosity and elasticity of a particular liquid crystal material.
                              τ          r                ∝                              γ            ⁢                                                  ⁢                          d              2                                            Δ            ⁢                                                  ⁢            ɛ            ⁢                                                                          V                  a                  2                                -                                  V                  F                  2                                                                                                      (        1        )            
Where, τr represents the rise time of a voltage applied to the liquid crystal material, Va represents the applied voltage, VF represents a Freederick Transition Voltage due to liquid crystal molecules starting to make a tilt motion, d represents a cell gap of a liquid crystal cell, and γ represents the rotational viscosity of a liquid crystal molecule.
                              τ          f                ∝                              γ            ⁢                                                  ⁢                          d              2                                K                                    (        2        )            
Where, τf represents the decay time during which a liquid crystal material molecule is restored to its original, untilted position by elastic restoration after the voltage applied to the liquid crystal material is removed, and K represents a unique elastic modulus of the liquid crystal material.
The response speed of the liquid crystal material of a twisted nematic TN mode, which is a very widely used liquid crystal mode in liquid crystal display devices up to now, can be changed in accordance with the physical properties and the cell gap of the liquid crystal material, but generally its rise time is about 20 ms˜80 ms and its decay time is about 20 ms˜30 ms. The response speed of such a liquid crystal material extends to the next frame before the voltage being applied to the liquid cell reaches a desired voltage, as shown in FIG. 1, because the response speed is longer than one frame period (NTSC: 16.67 ms). Thus a motion-blurring phenomenon occurs so that the LC device screen gets blurred when the device displays motion pictures.
Referring to FIG. 1, in a related art liquid crystal display device, the display brightness BL does not reach a desired brightness, so desired color and brightness are not able to be expressed, wherein the display brightness corresponds to the change of data VD from one level to another level due to a slow response speed. As a result, the liquid crystal display device causes a motion-blurring phenomenon to appear in motion picture displays and its picture quality lowers due to a deterioration of contrast ratio.
In order to rectify the slow response speed of the liquid crystal display device, U.S. Pat. No. 5,495,265 and PCT international publication No. WO 99/05567 have introduced a scheme (hereinafter ‘high speed driving method’) in which data are modulated in accordance with the existence or absence of a change of the data by use of a look-up table. The high speed driving method modulates data according to the principles illustrated in FIG. 2.
Referring to FIG. 2, the related art high speed driving method modulates input data VD into predetermined modulation data MVD and applies the modulated data MVD to a liquid cell to achieve a desired brightness MBL. The high speed driving method achieves the value of |Va2−VF2| in formula 1 on the basis of the existence or absence of a change of the data in order to achieve a desired brightness corresponding to the brightness value of the input data within one frame period. As a result, the liquid crystal display device using the high speed driving method compensates the slow response speed of liquid crystal material by modulating the data value to reduce the motion-blurring phenomenon in motion pictures display.
In other words, the high speed driving method compares data between a previous frame and a current frame. If the data is changed, then the high speed driving method modulates data of the current frame into the predetermined modulation data. A high speed driving apparatus implemented in this way can be implemented as in FIG. 3.
Referring to FIG. 3, the high speed driving method includes a first and a second frame memory 43a and 43b, respectively, for storing data from an input data line 42, and a modulator 44 to modulate data.
The first and the second frame memories 43a and 43b alternately store data of a frame unit in accordance with a pixel clock and then alternately output the stored data to supply a previous frame data, i.e., (n−1)th frame data fn−1 to the modulator 44.
The modulator 44 compares an (n)th frame data Fn from a data input line 43 and a (n−1)th frame data Fn−1 from the first and the second frame memories 43a and 43b, and then selects modulation data MRGB corresponding to the comparison result from a look-up table such as Table 1 to modulate the data. The look-up table may be stored in a read only memory ROM.
TABLE 101234567891011121314150023456791012131415151515101345678101213141515151520024567810121314151515153001356781011131415151515400134678911121314151515500123578911121314151515600123468910121314151515700123457910111314151515800123456810111214151515900123456791112131415151000123456781012131415151100123456789111314151512001234567891012141515130012334567810111315151400123345678911121415150001233456789111315
In table 1, the leftmost column represents the data of the previous frame Fn−1 and the uppermost row represents the data of the current frame Fn.
During an (n)th frame period, as represented by a solid line in FIG. 3, the (n)th frame data Fn is stored in the first frame memory 43a and is also supplied to the modulator 44 pursuant to the pixel clock. At the same time, for a (n)th frame period, the second frame memory 43b supplies the (n−1)th frame data Fn−1 to the modulator 44.
On the other hand, for a (n+1)th frame period, as represented by a dotted line in FIG. 3, the (n+1)th frame data Fn+1 is stored in the second frame memory 43b and is also supplied to the modulator 44 pursuant to the pixel clock. At the same time, for a (n+1)th frame period, the first frame memory 43b supplies the (n)th frame data Fn to the modulator 44.
As described above, the high speed driving method requires two frame memories 43a and 43b in order to alternately supply the previous frame data to the modulator 44. Because the frame memory is a cause of increasing circuit expense, a scheme capable of reducing the number of frame memories or the capacitance of the frame memory is desired.